I built a Firefox extension that keeps a calculator and unit converter one click away

dev.to JavaScript2026-11-04 23:390 views

Alt-Tab to a calculator app to do a quick conversion. Command-Tab back. That two-step interruption happens dozens of times a day if you work with numbers.

I built Quick Calculator & Unit Converter for Firefox — a toolbar popup that keeps a calculator and unit converter one click away without leaving your current tab.

What it does

Calculator — full keyboard support, history of previous calculations
Unit converter — length, weight, temperature, volume, speed, area, data
Currency converter — live exchange rates (optional, requires internet)
Toolbar popup — available on every page, never opens a new tab

Why a browser extension instead of a standalone app

The use case is always mid-task: reading an article with measurements you want to convert, checking a price in a foreign currency, quick math while filling a form.

A browser extension is contextually correct for this. It is there in the browser, where you already are, and it disappears when you click away.

The popup architecture

Firefox toolbar extensions are popup HTML pages. The popup opens on icon click, runs its JavaScript, and closes when focus leaves.

{ "browser_action": { "default_popup": "popup.html", "default_title": "Quick Calculator" } }

State (calculator history, last unit selection) is stored in browser.storage.local so it persists across popup opens.

Unit conversion: direct formulas, no lookup tables

Most unit converter apps use a lookup table of conversion factors. I used direct formulas for the common conversions — it is more readable and easier to audit:

const converters = { temperature: { celsius_to_fahrenheit: c => (c * 9/5) + 32, fahrenheit_to_celsius: f => (f - 32) * 5/9, celsius_to_kelvin: c => c + 273.15, kelvin_to_celsius: k => k - 273.15, }, length: { meters_to_feet: m => m * 3.28084, feet_to_meters: ft => ft / 3.28084, kilometers_to_miles: km => km * 0.621371, miles_to_kilometers: mi => mi / 0.621371, }, };